What is the ruling on feeling uneasy about a specific thing without being pessimistic about it, such as entering a particular door or seeing a certain person?

Discomfort upon seeing a person or entering a place does not fall under the forbidden pessimism, unless it leads to an action or abandonment. Indeed, ṭīrah (superstitious belief in omens) is shirk, and ṭīrah is only that which makes you proceed or hold back. One must rely on Allah and beware of taṭayyur (seeking omens) and pessimism.